1. Field of the Invention:
This invention relates to oscillators, particularly to phase locked oscillators to provide an agile frequency carrier with low phase fluctuations, L (f), for radar and communication applications.
2. Description of the Prior Art:
In electronic countermeasure technology, a delay line oscillator is used with limited success to remember an RF input signal. An input signal to the delay line oscillator will be amplified and coupled to the oscillator output. A portion of the output signal is fed through a delay line and then fed back by means of a switch to the oscillator amplifier along with the input signal. The input signal is then removed by means of a switch leaving the delayed feedback signal as the only input to the oscillator amplifier. The output of the oscillator will then be governed by the delayed feedback signal.
A delay line oscillator has a plurality of natural frequencies which are orderly spaced at intervals of 1/.tau. where .tau. is the total delay time in the feedback loop. When the error between the input frequency and one of the "natural" frequencies of a delay line oscillator is less than a certain value, the output frequency of oscillation very nearly coincides with the input frequency. The natural frequencies occur at those frequencies for which the incremental loop gain exceeds unity when the feedback loop is closed and, simultaneously, experience precisely 2 .pi. n phase shift where n is an integer. The natural frequencies are spaced at intervals of 1/.tau. where .tau. is the delay of the line plus the group delay of the remaining circuitry comprising the feedback arrangement. The set of natural frequencies can be shifted in absolute frequency through variation of the phase introduced by a phase shifter.
A delay line oscillator is commercially available for coverage from the 2 to 4 gigahertz band and can provide "memory" times on the order of 10 microseconds, utilizing a 0.1 microsecond electromagnetic delay line when the uncertainty between the input or priming frequency and a natural frequency is within a predetermined range. A very important element in the 2 to 4 gigahertz system is the amplifier limiter which exhibits negligible incremental gain variation over the 2 to 4 gigahertz band as a function of input signal level up to the point where 10 to 20 db of limiting occurs. When the feedback loop is closed in response to an input signal, and the input signal is removed from the input, phase runout occurs and the oscillator reverts to oscillation at the nearest natural frequency unless an incremental gain is larger at some natural frequency farther removed.
If a priming signal is continually applied to the delay line oscillator and is of a level sufficiently large to prevent regeneration of noise at other natural frequencies having larger incremental gain, the output frequency can be made to have a frequency accuracy approaching that of the priming signal. Reference is made to a publication by R. Adler, entitled "A Study of Locking Phenomena in Oscillators", found in the Proceedings of the IRE and Waves and Electrons, June 1946, pages 351 through 357, which discusses in detail the performance attainable in LC oscillators. Adler's results are applicable to a primed delay line oscillator when the Q in Adler's equations is replaced by the quantity .pi.f.sub.o .tau..
In an oscillator, a fundamental parameter that determines L (f) (the phase noise sideband spectrum) is the group delay d.phi./d.omega. of the feedback transmission network of the oscillator. Expressed as d.phi./df, the phase shifting property of the simple resonator that is usually employed as the principal frequency determining element is simply described by Equation (1): EQU d.phi./df = (2Q/f.sub.o) radians/hertz (1)
where d.phi. represents the change in phase; df represents the change in frequency; f.sub.o represents the average frequency of the oscillator; and Q represents the ratio of the reactance to the effective loaded resistance of a conventional L-C resonator's inductor. Equation (1) applies when the amplifier phase shift is negligible so that operation of the oscillator is very nearly the resonant frequency of the resonator. Equation (2) is applicable when the frequency control in an oscillator is provided by a delay line rather than an L-C resonator. EQU d.phi./df = -2.pi..tau. radians/hertz (2)
At the present time, with a frequency tuning band of 10%, it is difficult to realize d.phi./df exceeding 10.sup.-7 radians per hertz in a conventional L-band VCO that employs a bipolar transistor as an amplifier-limiter and a varactor tuned loaded transmission line resonator for selecting the frequency of oscillation. The limitation in attainable d.phi./df in the VCO results in serious limitations in the attainable L (f) from the VCO when multifrequency selection is to be provided by a phase locked VCO.
A d.phi./df of 10.sup.-6 radians per hertz would, however, adequately meet the L(f) requirement of both ground based low to medium PRF MTI radar carrier frequency supplies, as well as L (f) requirements for high duty factor (50%) high PRF MTI radar sets. With such stability, a stable phase locked loop with excellent transient settling behavior becomes feasible since the loop band width needed in the phase locked loop is only 50 to 100 kilohertz.
A significant measure of the noise content of a microwave signal is the indication of the phase power spectrum of the signal as provided by the parameter L (f) which is a frequency domain measure of phase fluctuations (noise, instability, modulation). L (f) is defined as the ratio of the power in one phase noise sideband, referred to the input carrier frequency, on a per hertz of bandwidth spectral density basis, to the total signal power, at Fourier frequency f from the signal's average frequency f.sub.o of the signal.
Therefore, in order to provide an agile frequency source having low L (f), it is desirable to utilize a phase locked oscillator which may realize a d.phi./df of 10.sup.-6 radians per hertz and which can oscillate at a plurality of frequencies. In addition, it is desirable that the stability and frequency of the phase locked oscillator may be maintained without controlling the temperature of any of the components in the oscillator.